Method and system wherein timeslots allocated for common control channels may be reused for user traffic

ABSTRACT

A method and system is disclosed wherein timeslots designated in a wireless communication system as Common Physical Channel (CPCH) timeslots may be reused for user traffic. A CPCH timeslot used in a first cell may be reused by a second cell, assuming the first and second cells transmit control information in different CPCH timeslots, for user traffic. The second cell is permitted to reuse the timeslot in which the first cell is transmitting control information so long as the second cell&#39;s reuse of that timeslot does not degrade reception of control information in the first cell.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims priority from U.S. provisional application No.60/485,762 filed on Jul. 9, 2003, which is incorporated by reference asif fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communication systems. Moreparticularly, the present invention relates to optimizing availableresources in wireless communication systems.

BACKGROUND

Wireless communication systems typically use a Broadcast Channel (BCH)to communicate control information to facilitate communications betweenwireless transmit/receive units (WTRUs) and the system. For example, theBCH is used to communicate to WTRUs information regarding the RadioAccess Network (RAN) as well as information specific to the cell, evenbefore the WTRUs are connected. In Time Division Duplex (TDD) typesystems, for example, the BCH is transmitted on the Primary CommonControl Physical Channel (PCCPCH). Wireless communication can also haveother common control channels which are transmitted on Common PhysicalChannels (CPCH). For example, in TDD systems, the Forward Access Channel(FACH) is mapped on the Secondary Common Control Physical Channel(SCCPCH). Both PCCPCH and SCCPCH are examples of CPCH. The same reservedtimeslots are typically used throughout a wireless communication systemfor transmitting the CPCH. It is noted that the term “CPCH timeslot” isused to refer to any timeslot that is used to transmit CPCH in thesystem.

Depending on the performance of the WTRUs as well as the RF isolationbetween cells, a TDD type system may be able to only use a singletimeslot throughout the system to transmit its CPCH or it may have touse more than one timeslot to allow neighboring cells to use differenttimeslots and thus ensure good CPCH reception. The use of more than onetimeslot to transmit a CPCH throughout a system is referred to as “CPCHtimeslot reuse.” On one hand, dedicating a certain number of timeslotsstrictly for purposes of transmitting the CPCH (i.e. forbidding theiruse for dedicated channels (DCH)) can lead to inefficient use of thespectrum that in turn translates into capacity loss. On the other hand,reusing CPCH timeslots to transmit DCH signals (i.e. user traffic or DCHtraffic) is not done since it leads to highly interfered CPCH signalswhich could result into CPCH reception problems for WTRUs in some areas.Poor CPCH reception has many negative impacts on wireless communicationsystems. For example, poor CPCH reception may result in extended timeperiods for WTRUs trying to access the system, degradation of key radioresource management functions such as handoffs and power control, andservice holes for the BCH and FACH.

In currently known wireless communication systems, a certain number oftimeslots are dedicated solely for transmitting the CPCH with no attemptto reuse those timeslot for DCHs (i.e. user traffic). Therefore, it isdesirable to have a method and system where timeslots used fortransmitting the CPCH may be reused for user traffic.

SUMMARY

The present invention is a method and system wherein timeslotsdesignated in a wireless communication system as Common Physical Channel(CPCH) timeslots may be reused for user traffic. A CPCH timeslot used ina first cell may be reused by a second cell, assuming the first andsecond cells transmit control information in different CPCH timeslots,for user traffic. The second cell is permitted to reuse the timeslot inwhich the first cell is transmitting control information so long as thesecond cell's reuse of that timeslot does not degrade reception ofcontrol information in the first cell.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a diagram of three cells within a wireless communicationsystem.

FIG. 2 is a method wherein timeslots used in a wireless communicationsystem for transmitting Common Physical Channels (CPCH), i.e. CPCHtimeslots, may be reused for user traffic.

FIG. 3 is a wireless communication system wherein timeslots used in awireless communication system for transmitting Common Physical Channels(CPCH), i.e. CPCH timeslots, may be reused for user traffic.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base station (BS)includes but is not limited to a Node-B, site controller, access pointor any other type of interfacing device in a wireless environment.Further, it is noted that, the notion of Common Physical Channel (CPCH)relates to transmission and/or reception of any type of controlinformation and encompasses all common physical channels includingPrimary Common Control Physical Channel (CPCH) on which the broadcastchannel (BCH) is transmitted and the Secondary Common Control PhysicalChannel (SCCPCH) on which the Forward Access Channel (FACH) istransmitted. When reference is made to CPCH timeslots, it is noted thatthe CPCH timeslots are the timeslots in which a CPCH is beingtransmitted. Further, when a cell is said to be handling user traffic,the cell may be transmitting, receiving, or transmitting and receivinguser traffic.

In order to ensure adequate CPCH reception, wireless communicationsystems may have to dedicate a plurality of timeslots for the CPCHthroughout the system. Allocating a plurality of timeslots as CPCHtimeslots allows one cell, say cell A, to transmit its CPCH in adifferent timeslot than a neighboring cell, say cell B in order toreduce the amount of intercell interference perceived by the WTRU tryingto detect the CPCH of one of the two cells. However, the CPCH timeslotused by cell A to transmit its CPCH is not used by cell B where cell Buses another CPCH timeslot to transmit its CPCH. As explained further inconnection with method 200, however, the present invention enables cellB to handle user traffic in the CPCH timeslot used by cell A, and viceversa. That is, cell A may handle user traffic in the CPCH timeslot usedby cell B.

Referring initially to FIG. 1, there is shown three cells 102, 104, 106.Assume, the wireless communication system to which cells 102, 104, 106belong has allocated timeslots 1, 2, and 3 for transmission of the CPCH.That is, timeslots 1, 2, and 3 are CPCH timeslots. Further assume thatcell 102 is transmitting its CPCH in timeslot 1, cell 104 istransmitting its CPCH in timeslot 2, and cell 106 is transmitting itsCPCH in timeslot 3.

According to the present invention, a particular cell may reuse CPCHtimeslots used by other cells to transmit their CPCH, for purposes ofhandling user traffic in the particular cell, assuming the CPCHtimeslots being used by the other cells to transmit CPCH are differentthan the CPCH timeslot being used by the particular cell to transmit itsown CPCH. That is, taking cell 102 as an example, cell 102 is able tohandle user traffic (i.e. DCH traffic) in timeslots 2 and 3 at aparticular power level that will not result in unacceptable CPCHperformance degradation in cells 104 and 106. Cells 104 and 106 willpermit cell 102 to reuse their CPCH timeslots for user traffic so longas such reuse does not result in degradation of CPCH performance fortheir own users. The power level at which one cell may handle usertraffic in a CPCH timeslot being used by another cell to transmit itsCPCH is denoted P_(max) _(—) _(dch) _(—) _(cpch).

To further explain, assume cell 102 is reusing the CPCH timeslot used bycell 104 for CPCH, which as explained above is timeslot 2, for usertraffic. Cell 104 will allow cell 102 to use timeslot 2 for user trafficso long as cell 102's use of timeslot 2 does not result in degradationof CPCH performance in cell 104. This requires the system to perform thefollowing actions: monitor CPCH performance in each cell, identify anyCPCH performance degradation in a cell due to reuse of the CPCH timeslotby other cells to transmit user traffic, and finally identify thecell(s) responsible for potential CPCH performance degradation andensure that the adequate CPCH performance level is restored. There aremany ways in which a cell may monitor CPCH performance. For example, thesystem may collect, in each cell, CPCH quality metrics reported by eachmobile. The metrics are preferably collected by base stations (BSs)operating within the system.

Examples of CPCH quality metrics specific to the Primary Common ControlPhysical Channel (PCCPCH), for example, include but are not limited toBCH reading time and Signal-to-Interference Ratios (SIR) perceived by aWTRU on the PCCPCH. Similarly, examples of CPCH quality metrics specificto the SCCPCH include but are not limited FACH Block Error Rate (BLER),FACH Bit Error Rate (BER), and Signal-to-Interference Ratios (SIR)perceived by a WTRU for the SCCPCH. Each CPCH quality metric collectedby a cell is preferably associated with a specific area of the cell. Anarea of a cell can be represented as an angular section of the cell orany arbitrary division of the overall geographical area of the cell. Inorder for the BS of a cell to associate each CPCH quality metric itcollects to a specific area of a cell, it has to be able to locate theposition of the WTRU which reported the CPCH quality metric.

Possible ways in which the system can identify the location of the WTRUinclude but are not limited to the use of Global Positioning Systems(GPS) in the WTRU and triangulation techniques based on delay ofarrivals, or measured power from neighboring BS. As each cell in thesystem is able to collect CPCH quality metrics from a large number ofWTRUs and associate them to specific areas of the cell, the system isable to obtain, for each area of each cell, a distribution of the CPCHquality metric. An example of the form that could take this distributionis a histogram in which each bin would correspond to a small interval ofthe quality metric.

Prior to the system trying to reuse the CPCH timeslots for user traffic,the system collects enough statistics from the WTRUs to obtainstatistically stable distributions for each area of each cell. Thesedistributions are referred to as baseline CPCH quality distributions andwill be used by the system as a comparison benchmark in order toidentify any degradation in CPCH quality in any area of any cell. If thesystem identifies an area of a cell where CPCH performance is degraded,the system identifies the cell responsible for the interference andreduces this interference to a level which would restore the previousstate where CPCH quality was deemed acceptable. To achieve this, thesystem preferably uses a database containing a pre-determined mappingwhich associates each area of each cell with its strongest interferingcell(s). Therefore, where cell 104 identifies area 108, for example, asbeing the area of unacceptable CPCH performance, it is evident that thecause of the degradation is cell 102's reuse of timeslot 2 for usertraffic. In this case, in cell 102, timeslot 2 is identified asaggressive, meaning reuse of timeslot 2 by cell 102 has resulted indegradation of CPCH performance in the cell 104 which is using timeslot2 to transmit its CPCH. Therefore, cell 102 has to decrement the powerit is using for user traffic in timeslot 2 and is no longer able to tryto increase the power at which it reuses timeslot 2 for user traffic. Itis noted that cell 102 may have timeslot 2 tagged as aggressive whileother cells such as, for example 106, may have timeslot 2 tagged asnon-aggressive. That is, timeslot 2 may be considered aggressive withrespect to cell 102, but not cell 106 meaning cell 106 can still reusetimeslot 2 for user traffic.

Referring now to FIG. 2, there is shown a method 200 wherein timeslotsused in a wireless communication system for transmitting the CPCH (i.e.CPCH timeslots) may be reused for user traffic. It is noted that method200 may be implemented in any number of cells as desired. Method 200begins with step 202 where, for each cell a tag is placed on the CPCHtimeslots that the cell is not using to transmit its own CPCH. The tagidentifies CPCH timeslots as being non-aggressive, meaning they are notcausing degradation of another cell's CPCH performance. Also, in step202, for each cell, the power at which the cell is permitted to transmituser traffic in a CPCH timeslot (i.e. P_(max) _(—) _(dch) _(—) _(cpch))is set to zero for all CPCH timeslots. That is, for each cell, theP_(max) _(—) _(dch) _(—) _(cpch) of each CPCH timeslot is set to zero.Further, in step 202, the system collects CPCH quality metrics for eacharea of each cell, thus obtaining statistically stable baselinedistributions that will be used as benchmarks in step 214.

From step 202, the method 200 proceeds to step 204 where, for each cell,a tag is placed on the CPCH timeslot that the cell is using to transmitits own CPCH as aggressive. This will prevent a cell from handling usertraffic in a CPCH timeslot that the cell is using itself fortransmission of the CPCH. In step 206, it is determined whether allcells have all their CPCH timeslots either set as aggressive or havetheir P_(max) _(—) _(dch) _(—) _(cpch) set to P_(max) where P_(max)corresponds to the maximum power a BS is allowed or able to transmit ina timeslot. For example, P_(max) for a BS allowed or able to transmit upto 43 dBm is 43 dBm.

If the result of step 206 is yes, the method 200 ends in step 208. Byway of explanation, when a cell has a CPCH timeslot tagged asaggressive, it indicates that the cell is already transmitting at apower beyond which it would degrade the CPCH reception of at least oneof its neighboring cells. When a cell has a CPCH timeslot for whichP_(max) _(—) _(dch) _(—) _(cpch) is set to P_(max), it indicates thatthe cell is already fully reusing this CPCH timeslot for user traffic.Therefore, if either of the above-mentioned conditions are fulfilled forall CPCH timeslots of all cells, the system is in a state where cellsare not able to further increase the reuse of CPCH for user traffic. Inother words, the system is in a state where cells are not able tofurther increase P_(max) _(—) _(dch) _(—) _(cpch) of any of their CPCHtimeslots and the method 200 ends.

If the result in step 206 is no, the method 200 proceeds to step 210. Instep 210, for each cell, the P_(max) _(—) _(dch) _(—) _(cpch) of eachCPCH timeslot that is not tagged as aggressive and has its P_(max) _(—)_(dch) _(—) _(cpch) set lower than P_(max), is incremented by in apredetermined amount, say P_(—)increment. Then, in step 212, the systemcollects measurements on CPCH performance and obtains, for each area ofeach cell, distributions of CPCH quality metrics.

In step 214, it is determined whether the CPCH performance isunacceptable in any area of any cell. This is accomplished by comparingthe distribution of CPCH quality measurement collected for every area ofevery cell with the baseline distributions collected in step 202 andidentifying any area having unacceptable quality measurements. If no,the method 200 returns to step 206. If yes, the method 200 proceeds tostep 218. Then, the cell(s) that are causing the CPCH performancedegradation is identified (step 218). This is accomplished by looking upa database containing a predetermined mapping which associates each areawithin each cell to their strongest interfering cell(s) so that wheredegradation is identified in a particular area, the system knows who theoffending cell(s) is (are). For example, referring back to FIG. 1, area108 is mapped to cell 102.

In step 220, P_(max) _(—) _(dch) _(—) _(cpch) of the offending cell(s)is decremented by P_(—)increment for the CPCH timeslot where CPCHperformance degradation was measured and that CPCH timeslot is tagged asaggressive with respect to the offending cell(s) identified in step 218.Once step 220 is complete, the method 200 returns to step 206.

Referring now to FIG. 3, there is shown a wireless communication system300 wherein timeslots used in a wireless communication system fortransmitting the CPCH (i.e. CPCH timeslots) may be reused for usertraffic. The system includes at least one radio network controller (RNC)302 and a plurality of cells 304, 306, 308. In this embodiment, thesystem 300 is shown as being deployed with an omnidirectional deploymentwherein there is a BS 305, 307, 309 for each cell 304, 306, 308. Thesystem 300 could of course be deployed with a sectored deployment,wherein a single BS is provided for cells 304, 306, 308.

As explained above, a plurality of timeslots are typically designated asCPCH timeslots and are used by the cells for transmitting the CPCH.Assume, in this embodiment that three timeslots 1, 2, 3, have again beendesignated as CPCH timeslots for system 300. Therefore, all of the cellsmaking up system 300 will transmit their CPCH in one of the three CPCHtimeslots. For simplicity, only three cells 304, 306, 308 of system 300are shown, but of course system 300 may have any number of cells asdesired. Because there are only three cells, each cell may use adifferent CPCH timeslot for transmitting its CPCH. Where there are morecells, they will share the allocated CPCH timeslots in the same manner.That is, where there are ninety cells and three CPCH timeslots, forexample, each of the ninety cells will use one of the three CPCHtimeslots for transmitting its CPCH.

In system 300, assume cell 306 is transmitting its CPCH in CPCH timeslot1, cell 304 is transmitting its CPCH in CPCH timeslot 2, and cell 308 istransmitting its CPCH in timeslot 3. For each area of each cell, CPCHperformance is monitored and if it becomes unacceptable, the area withinthe cell where the unacceptable CPCH is concentrated is identified.Therefore, the BS 305, 307, 309 of cells 304, 306, 308 each include aprocessor 310, 312, 314 for collecting CPCH readings or any other metricof CPCH performance from WTRUs operating within their cell. Where CPCHis identified as being unacceptable in any of the cells, the location ofthe WTRUs that are reporting the poor CPCH measurements is identified.The BS 305, 307, 309 of cells 304, 306, 308 may each include a separateprocessor 316, 318, 320 for locating WTRUs, or that functionality may beperformed in processors 310, 312, 314.

The RNC 302 to which data collected in each cell is reported alsoincludes at least one processor 322 for determining when CPCH hasdegraded to an unacceptable level and coordinating each cells reuse ofCPCH timeslots for user traffic. In coordinating each cells reuse ofCPCH timeslots for user traffic, the RNC 302 will inform each cell atwhich P_(max) _(—) _(dch) _(—) _(cpch) they may transmit user traffic,if at all, in the CPCH timeslots being used by their neighboring cellsto transmit CPCH. Where CPCH performance has degraded to an unacceptablelevel in a particular area of any particular cell, say cell 306, as aresult of another cell's, say cell 308, reuse of the CPCH timeslot cell306 is using to transmit its CPCH (i.e. CPCH timeslot 1), the RNC 302will ensure the BS 309 of cell 308 decreases the power at which it isreusing timeslot 1 for user traffic back to a level which does notimpair CPCH performance in cell 306. The RNC 302 will preferably preventcell 308 from further increasing the power (i.e. P_(max) _(—) _(dch)_(—) _(cpch)) that is used for user traffic in CPCH timeslot 1.

It is important to note that the present invention may be implemented inany type of wireless communication system employing any type of timedivision duplex (TDD) technology, as desired. By way of example, thepresent invention may be implemented in UMTS-TDD, TDSCDMA, or any othertype of wireless communication system. Further, while the presentinvention has been described in terms of various embodiments, othervariations, which are within the scope of the invention as outlined inthe claim below will be apparent to those skilled in the art.

1. A method for reusing timeslots designated for transmission of controlinformation for user traffic, the method comprising the steps of:identifying timeslots designated for transmission of control informationwithin a wireless communication system including a plurality of cells;allocating the timeslots designated for transmission of controlinformation throughout the plurality of cells so that a time slotallocated for transmission of control information in a first cell is notused for transmission of control information in any cells neighboringsaid first cell; and wherein said time slot allocated for transmissionof control information in the first cell is reused for user traffic incells neighboring the first cell.
 2. The method of claim 1 wherein thetimeslots designated for transmission of control information are CommonPhysical Channel (CPCH) timeslots.
 3. The method of claim 2 wherein thefirst cell is identified as the cell causing degradation to CPCHreception in the second cell by locating an area within the second cellin which wireless transmit/receive units (WTRUs) have reported poor CPCHreception and poor CPCH reception in said area indicates interferencefrom the second cell.
 4. The method of claim 2 wherein a Primary CommonControl Physical Channel (PCCPCH) is transmitted in the CPCH timeslots.5. The method of claim 2 wherein a Secondary Common Control PhysicalChannel (SCCPCH) is transmitted in the CPCH timeslots.
 6. The method ofclaim 1 further comprising the step of: reducing the power with which afirst cell reuses a CPCH timeslot for user traffic where the reusecauses degradation to CPCH reception in a second cell using said CPCHtimeslot for transmission of control information.
 7. The method of claim6 wherein cells other than the first cell that are not using the CPCHtimeslot for transmission of control information reuse the CPCH timeslotfor user traffic.
 8. A method for reusing Common Physical Channel (CPCH)timeslots for user traffic within a first cell of a wirelesscommunication system, the system including a plurality of cells and eachcell transmitting control information in an allocated CPCH timeslot, themethod comprising: transmitting and receiving user traffic in CPCHtimeslots which are utilized for transmission of control information incells neighboring said first cell.
 9. A wireless communication systemwherein timeslots designated for transmission of control information maybe reused for user traffic, the system comprising: a plurality ofwireless transmit/receive units (WTRUs) configured to report performancemeasurements regarding the quality with which they receive controlinformation; at least one base station configured to receive theperformance measurements provided by the WTRUs and obtain the locationof each WTRU reporting the performance measurements; and a radio networkcontroller (RNC) configured to identify base stations whose reuse oftimeslots designated for transmission of control information isdegrading reception of control information at another base station andadjust the power with which said base stations are reusing saidtimeslots.
 10. The wireless communication system of claim 9 wherein theRNC is configured to reduce the power with which a first cell reuses aCPCH timeslot for user traffic where the reuse causes degradation toCPCH reception in a second cell using said CPCH timeslot fortransmission of control information.
 11. The wireless communicationsystem of claim 10 wherein the RNC identifies the first cell as the cellcausing degradation to CPCH reception in the second cell by locating anarea within the second cell in which WTRUs have reported poor CPCHreception wherein poor CPCH reception in said area indicatesinterference from the second cell.
 12. The wireless communication systemof claim 11 wherein the RNC allows cells other than the first cell thatare not using the CPCH timeslot for transmission of control informationto reuse the CPCH timeslot for user traffic.